diff --git a/src/agents/anthropic-transport-stream.test.ts b/src/agents/anthropic-transport-stream.test.ts
index 3d546ac0d7f..ca0c7abbdb8 100644
--- a/src/agents/anthropic-transport-stream.test.ts
+++ b/src/agents/anthropic-transport-stream.test.ts
@@ -308,6 +308,7 @@ describe("anthropic transport stream", () => {
       } as AnthropicStreamContext,
       {
         apiKey: "sk-ant-api",
+        toolChoice: { type: "tool", name: "read_file" },
       } as AnthropicStreamOptions,
     );
 
@@ -2121,6 +2122,7 @@ describe("anthropic transport stream", () => {
     const payload = latestAnthropicRequest().payload;
     expect(payload.thinking).toEqual({ type: "adaptive" });
     expect(payload.output_config).toEqual({ effort: "high" });
+    expect(payload.tool_choice).toEqual({ type: "auto" });
   });
 
   it("does not infer adaptive thinking from forward-compatible effort maps", async () => {
@@ -2302,6 +2304,33 @@ describe("anthropic transport stream", () => {
     expect(payload.output_config).toEqual({ effort: "high" });
   });
 
+  it("uses mandatory adaptive thinking for canonical Claude Mythos Preview transport aliases", async () => {
+    const model = makeAnthropicTransportModel({
+      id: "prod-mythos-preview",
+      name: "Production Claude",
+      provider: "microsoft-foundry",
+      params: { canonicalModelId: "claude-mythos-preview" },
+      reasoning: false,
+      baseUrl: "https://example.services.ai.azure.com/anthropic",
+      maxTokens: 128_000,
+    });
+
+    guardedFetchMock.mockResolvedValueOnce(createSseResponse());
+    await runTransportStream(
+      model,
+      {
+        messages: [{ role: "user", content: "Think." }],
+      } as AnthropicStreamContext,
+      {
+        apiKey: "sk-ant-api",
+      } as AnthropicStreamOptions,
+    );
+
+    const payload = latestAnthropicRequest().payload;
+    expect(payload.thinking).toEqual({ type: "adaptive" });
+    expect(payload.output_config).toEqual({ effort: "high" });
+  });
+
   it("maps Claude Fable 5 transport thinking levels to adaptive effort", async () => {
     const model = makeAnthropicTransportModel({
       id: "claude-fable-5",
diff --git a/src/agents/anthropic-transport-stream.ts b/src/agents/anthropic-transport-stream.ts
index 4812b028b23..f8887511c5f 100644
--- a/src/agents/anthropic-transport-stream.ts
+++ b/src/agents/anthropic-transport-stream.ts
@@ -17,6 +17,7 @@ import type {
 import { parseStreamingJson } from "../llm/utils/json-parse.js";
 import {
   resolveClaudeNativeThinkingLevelMap,
+  requiresClaudeAdaptiveThinking,
   supportsClaudeAdaptiveThinking,
   supportsClaudeNativeMaxEffort,
   supportsClaudeNativeXhighEffort,
@@ -141,7 +142,7 @@ function normalizeAnthropicToolChoice(
   toolChoice: AnthropicTransportOptions["toolChoice"],
 ) {
   if (
-    usesClaudeFable5MessagesContract(model) &&
+    requiresClaudeAdaptiveThinking(model) &&
     (toolChoice === "any" || (typeof toolChoice === "object" && toolChoice.type === "tool"))
   ) {
     return { type: "auto" as const };
@@ -1025,7 +1026,7 @@ function resolveAnthropicTransportOptions(
     reasoning: options?.reasoning,
   };
   if (!options?.reasoning) {
-    resolved.thinkingEnabled = usesClaudeFable5MessagesContract(model);
+    resolved.thinkingEnabled = requiresClaudeAdaptiveThinking(model);
     if (resolved.thinkingEnabled) {
       resolved.effort = "high";
     }
diff --git a/src/llm/providers/anthropic.test.ts b/src/llm/providers/anthropic.test.ts
index 4e1b3e96f7d..a07f14d3092 100644
--- a/src/llm/providers/anthropic.test.ts
+++ b/src/llm/providers/anthropic.test.ts
@@ -537,6 +537,38 @@ describe("Anthropic provider", () => {
     });
   });
 
+  it("uses mandatory adaptive thinking for Foundry Mythos Preview", async () => {
+    let capturedPayload: unknown;
+    const stream = streamSimpleAnthropic(
+      makeAnthropicModel({
+        id: "prod-mythos-preview",
+        name: "Production Claude",
+        provider: "microsoft-foundry",
+        params: { canonicalModelId: "claude-mythos-preview" },
+        reasoning: false,
+      }),
+      {
+        messages: [{ role: "user", content: "hello", timestamp: 0 }],
+      },
+      {
+        apiKey: "sk-ant-provider",
+        toolChoice: "any",
+        onPayload: (payload) => {
+          capturedPayload = payload;
+          throw new Error("stop before network");
+        },
+      },
+    );
+
+    await stream.result();
+
+    expect(capturedPayload).toMatchObject({
+      thinking: { type: "adaptive" },
+      output_config: { effort: "high" },
+      tool_choice: { type: "auto" },
+    });
+  });
+
   it("uses adaptive high effort for Foundry Mythos Preview without native max metadata", async () => {
     let capturedPayload: unknown;
     const stream = streamSimpleAnthropic(
diff --git a/src/llm/providers/anthropic.ts b/src/llm/providers/anthropic.ts
index 71d759edc42..25d56bd933a 100644
--- a/src/llm/providers/anthropic.ts
+++ b/src/llm/providers/anthropic.ts
@@ -14,6 +14,7 @@ import {
 } from "../../agents/system-prompt-cache-boundary.js";
 import {
   resolveClaudeNativeThinkingLevelMap,
+  requiresClaudeAdaptiveThinking,
   supportsClaudeAdaptiveThinking,
   supportsClaudeNativeMaxEffort,
   supportsClaudeNativeXhighEffort,
@@ -802,7 +803,7 @@ function normalizeAnthropicToolChoice(
   toolChoice: AnthropicOptions["toolChoice"],
 ) {
   if (
-    usesClaudeFable5MessagesContract(model) &&
+    requiresClaudeAdaptiveThinking(model) &&
     (toolChoice === "any" || (typeof toolChoice === "object" && toolChoice.type === "tool"))
   ) {
     return { type: "auto" as const };
@@ -875,11 +876,11 @@ export const streamSimpleAnthropic: StreamFunction<"anthropic-messages", SimpleS
 
   const base = buildBaseOptions(model, options, apiKey);
   if (!options?.reasoning) {
-    const fable5 = usesClaudeFable5MessagesContract(model);
+    const mandatoryAdaptiveThinking = requiresClaudeAdaptiveThinking(model);
     return streamAnthropic(model, context, {
       ...base,
-      thinkingEnabled: fable5,
-      ...(fable5 ? { effort: "high" as const } : {}),
+      thinkingEnabled: mandatoryAdaptiveThinking,
+      ...(mandatoryAdaptiveThinking ? { effort: "high" as const } : {}),
     } satisfies AnthropicOptions);
   }
 
diff --git a/src/shared/anthropic-model-contract.ts b/src/shared/anthropic-model-contract.ts
index 700c8832a10..749681bf054 100644
--- a/src/shared/anthropic-model-contract.ts
+++ b/src/shared/anthropic-model-contract.ts
@@ -49,6 +49,21 @@ export function usesClaudeFable5MessagesContract(model: {
   );
 }
 
+export function requiresClaudeAdaptiveThinking(model: {
+  id?: string;
+  params?: Record<string, unknown>;
+  api?: string;
+}): boolean {
+  if (normalizeApi(model.api) !== "anthropic-messages") {
+    return false;
+  }
+  const modelId = resolveClaudeModelIdentity(model);
+  return (
+    resolveClaudeFable5ModelIdentity(model) !== undefined ||
+    /(?:^|-)claude-mythos-preview(?=$|[^a-z0-9])/.test(modelId)
+  );
+}
+
 function resolveReplayFableIdentity(ref: ReplayModelRef): string | undefined {
   if (normalizeApi(ref.api) !== "anthropic-messages") {
     return undefined;